Sales at stores open at least a year climbed 3 percent. In the U.S, the metric also rose 3 percent. It edged up 1 percent overseas.

This figure is a key gauge of a retailer's health because it excludes results from locations recently opened or closed.

Taking out the impact of lower gas prices and foreign currency exchange fluctuations, sales at stores open at least a year increased 5 percent. The metric rose 4 percent in the U.S. and climbed 6 percent abroad.

Costco currently runs 648 warehouses, including 461 in the U.S. and Puerto Rico, 87 in Canada, 33 in Mexico, 25 in the U.K., 18 in Japan, 10 in Taiwan, nine in Korea and five in Australia.

The company's stock dropped $5.21, or 4.3 percent, to $114.83 in premarket trading three hours before the market open.